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Preparation for an introduction of African Horse Sickness: where do Culicoides midges land?

Gepubliceerd op
20 september 2016

At the moment there is no direct threat, but if African horse sickness (AHS) is introduced into western Europe, we have to protect horses and ponies against this disease. Culicoides midges, small biting insects, play an important role in the transmission of the virus that causes AHS.

Protection against AHS can be obtained by means of vaccination. However, the live-attenuated vaccine that is available in Africa will probably not be used in Europe because of the risk of spread of vaccine virus in the field.  Because of that, a new generation (not-live-attenuated) of vaccines against AHS is developed in Europe, but those vaccines are not yet ready for the market. In this view it is important to consider other types of measure to prevent the horses and ponies to be bitten by midges.

Results of a study by Wageningen Bioveterinary Research* (WBVR) in Lelystad indicate that midges are only active at specific hours of the day and where they land on the skin of a horse to take a blood meal. During an AHS-outbreak in the Netherlands in the midge-active season (May-October), horses and ponies can be best protected by a protective blanket, hoodie and leg bandages during the day – except during a cloudless period between 11:00 – 16:00 hours when there is no midge activity. During the night it is advised to stable the animals in a roofed structure, all openings sealed with a finely meshed screen. The study was recently published in the scientific journal  Entomologia Experimentalis et Applicata

Epidemiologist dr. Armin Elbers (Wageningen Bioveterinary Research) and student Sammy-Jo van den Heuvel (CAH Vilentum, Dronten) caught large quantities of midges during the summer up and around Fjord horses, especially for this study provided by Ruitersportcentrum Harderwijk.
We do basic research concerning midges that are associated with transmission of viruses such as bluetongue and Schmallenberg virus in livestock and AHS virus in horses and ponies. AHS is endemic in sub Sahara Africa. Occasionally it emerges to more northern areas, even into southern Europe. For instance, between 1987 and 1991 more than a thousand horses died during an epidemic of AHS in Spain, Portugal and Morocco. We cannot rule out that AHS might strike in north western Europa and we must therefore investigate how we can prepare optimally for a future introduction of AHS in the Netherlands and the rest of NW Europe.

For this study, the body of the horse was divided into 11 regions. Culicoides spp. were landing on all parts of the body. The Culicoides spp. that were found most frequently were Culicoides chiopterus, C.punctatus, C.obsoletus complex and C.dewulfi, all proven or potential vectors for arboviral diseases.

Culicoides spp. activity was distinctly bimodal across the day, surging at sunset and 1 h after sunrise. Midges were inactive between 11:00 and 16:00 hours, these hours marking the time of day when horses can be pastured most safely but, thereafter, to avoid escalating attacks, would have to be stabled protectively. Around sunset, the mean attack rate of the four most abundant species ranged from 3.0 to 11.7 midges per min; of these, C.dewulfi and C.chiopterus were reared out of the dung of experimental horses.

The Netherlands is home to the world's densest horse population (11 per km2), of which half are estimated to stay outdoors permanently with no access to protective housing. In the absence of a preventive vaccination policy, it is difficult to envisage how horses in Northern Europe will be protected from infection during an outbreak of a Culicoides-transmitted disease like African horse sickness.B

* Wageningen Bioveterinary Research is vanaf 6 september 2016 de naam van Central Veterinary Institute (CVI)